Huntington's disease (HD) is the most prevalent autosomal dominant, trinucleotide repeat neurodegenerative disease. The huntingtin (htt) gene encodes a protein of 350 kD;the disease-causing mutation causes an expansion of an amino-terminal polyglutamine repeat of more than 36 successive glutamines. Our broad research goal is to understand the molecular basis of HD pathogenesis, to target therapy. Because Huntington's disease is an inherited disease, we expect that the mutant allele will differ from wild-type by at least a single nucleotide polymorphism (SNP). Our core idea is that RNA silencing could be used) to selectively reduce mutant htt production and thereby slow or block neuronal dysfunction and death in HD disease. We propose 5 Aims. Aim 1 will identify all the SNPs present in the htt mRNA, to identify those SNP9 to which SNP-selective siRMAs can be designed. Aim 2 will develop SNP-selective siRNAs and then study their efficacy in cell models: luciferase reporter assays in HeLa cells, X-57 immortalized neuronal cells transfected with human htt, and primary striatal and cortical neurons expressing human htt through lentiviral transduction. Effects of htt RNAi on molecular correlates of HD pathogenesis (htt fragment accumulation, htt aggregation and autophagy) will be measured. Aim 3 will examine the effectiveness of siRNAs directed against htt mRNA that are not SNPselective. Off-target effects of htt siRNAs will be studied. Aim 4 will pursue an alternative strategy to block. htt mRNA, by creating a molecular tether that recruits miRNA to target rriRNAs. Aim 5 will iesi the most active, SNP-selective siRNA in mouse models of Huntington's disease, through delivery of conjugated htt siRNAs and lentivirus htt shRNA. HD molecular correlates, neurophysiology and behavior will be measured. Preliminary and published studies support each of the aims, including development of htt SNP-selective hyper functional siRNAs, delivery of siRNAs to primary neurons, and molecular and whole animal endpoints. These aims are expected to provide HD therapy and insights into molecular correlates that mark its pathogenesis.